This invention is directed to an automatic fill-stop valve for automatically stopping the filling of a propane cylinder when it reaches the desired maximum level. The valve is particularly useful for the portable tanks used in recreational vehicles and other small installations which are called cylinders. These cylinders and fittings are controlled by the regulations of the Department of Transportation, and are often known as DOT cylinders.
Liquefied petroleum gas, including propane and butane, is extensively used for household cooking and heating where natural gas or manufactured gas is unavailable. Furthermore, liquefied petroleum gas is used for these purposes. Liquefied petroleum gas is extremely volatile and burns readily. In the liquefied state, the vapor pressure of propane and butane is such that the liquid can be stored in tanks of only moderate strength in outdoor locations at the ambient temperatures encountered. These characteristics make liquefied petroleum gas extremely desirable for use as fuels. In addition to cooking and space heating, the gases are employed as internal combustion engine fuel and as process gas.
Liquefied petroleum gas must be handled very cautiously due to its characteristics. For example, liquefied propane and butane have boiling points lower than the normal atmospheric temperatures. Thus, they vaporize readily under normal ambient temperature conditions. The vapor pressure is sufficiently high that they are volatile at ordinary outdoor ambient temperatures, at atmospheric pressure. However, the pressure can be contained in the tank in reasonable thickness so that the volatility provides the tank pressurization which supplies the gas to its point of use.
In addition to the foregoing characteristics, these liquefied petroleum gases have extremely high thermal coefficients of expansion such that even moderate changes in temperature thereof cause the liquid to expand or contract appreciably. For this reason, it is necessary to maintain a void or head space in a tank where the liquefied petroleum gas is stored. A portion of the volume of any such tank must be left unoccupied with the liquid to allow for expansion upon increase in temperature. In fact, most states now prohibit the filling of any such tank with liquefied gas beyond a certain percentage of the volume of the tank.
The most common present device for determining maximum tank fill comprises a level-sensing dip tube which extends down into the tank to the point of highest proper liquid level in the tank. The dip tube vents outside the tank and has a manual valve thereon. The manual valve is opened, and filling is commenced. Vapor from the tank head space is discharged out of the open manual valve and, when the tank is filled with liquid to the bottom of the dip tube, the liquid level is sensed thereat by the liquid discharging from the manual valve. The liquid expands and vaporizes as it reaches the atmosphere, thus creating a telltale white cloud. When seeing such a cloud, the operator shuts off the fill valve and terminates the filling operation. Another common fill-limit detection means is weighing.
Of course, such a system requires that the filling operator remain alert, because he must turn off the fill valve at the proper time. If the operator fails to respond at the right time, the tank can be filled considerably above the safe limit. Thereafter, in the event of even a moderate increase in temperature of the liquid, the liquid can expand to fill the entire tank. Further expansion would cause a rapid increase in pressure and could rupture the tank. With tank rupture, an extremely hazardous condition occurs when the volatile liquid and vapor is spilled. A spark can produce a destroying fire.
To prevent the occurrence of such hazardous conditions, it is apparent that an automatic fill-stop valve is required. G. D. Mylander U.S. Pat. No. 3,363,641 illustrates an automatic flow control valve which is responsive to the liquid level so that filling is stopped. This prior automatic flow control valve is arranged for ASME tanks with separate filling and withdrawal ports. That filling-stop valve is not adaptable to cylinders.